Backlight Device and Display Device

ABSTRACT

It is an object to manufacture a highly reliable backlight device with less color unevenness and less luminance unevenness, and a high-performance and highly reliable display device including the backlight device, which can display a high quality image. A light emitting diode (LED) is used as a light source of a backlight device and thermoelectric elements are provided in a chassis for holding the light emitting diode so as to surround the light emitting diode (the thermoelectric elements are provided under the light emitting diode and on the four sides thereof). A temperature in the backlight device is adjusted by cooling and heating by the thermoelectric elements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight device, and a displaydevice including a liquid crystal element, provided with the backlightdevice.

2. Description of the Related Art

Nowadays, a liquid crystal display device is used in a wide range offields such as a liquid crystal television, a PDA, a mobile phone, andoffice automation equipment like a personal computer, as well as a clockand a calculator.

In a liquid crystal display device, liquid crystal is sealed between twolight-transmitting substrates. When voltage is applied, a direction ofliquid crystal molecules is changed and light transmittance thereof ischanged, so that a predetermined image or the like is displayedoptically. Since liquid crystal does not emit light by itself, abacklight unit serving as a light source is provided on the back of aliquid crystal display panel. The backlight unit includes a lightsource, a light guide plate, a reflective film, a prism film, adiffusing film, and the like, and supplies light to display an image orthe like to the entire surface of the liquid crystal display paneluniformly.

As the light source in the backlight unit, a cold cathode fluorescentlamp in which mercury or xenon is sealed in a fluorescent tube isgenerally used.

A light source in a backlight device as described above has luminancewhich changes according to an ambient temperature and there is a casesuch that luminance of the light source is decreased because of rise intemperature due to its own heat generation. Therefore, in order to makea temperature of the light source an optimum driving temperature, ameasure for releasing heat such as a heat sink, a heat pipe, or air coolby a cooling fan is tried. As one of measures for releasing heat, amethod in which a fluorescent tube has a Peltier element and cooling iscarried out by the Peltier element is given (for example, see PatentDocument 1: Japanese Published Patent Application No. H6-324304 andPatent Document 2: Japanese Published Patent Application No. H7-175035).

SUMMARY OF THE INVENTION

A white light emitting diode or high output. RGB light emitting diodesare used recently, and high output RGB light emitting diodes have acharacteristic such that a driving voltage thereof is high and luminanceis low when being driven at high temperature. In particular, a red lightemitting diode largely depends on a temperature. Further, in a lightemitting diode backlight unit, when a plurality of high output RGB lightemitting diodes are used, a large amount of heat is generated.

Due to such heat generation, reduction in life time and luminance,chromaticity shift, and the like are occurred in the light emittingdiode, and transformation, alteration, and the like of a diffusing film,a reflective film, a prism film, and the like are occurred in thebacklight unit.

Further, since a liquid crystal display panel is located in front of thebacklight unit, a temperature of the backlight unit influences theliquid crystal display panel to no small extent. Accordingly, effect oncharacteristics of the liquid crystal display panel, such as responsespeed, contrast, or color unevenness; and transformation, alteration,deterioration in characteristics, or the like of a polarizing film, awide view film, a retardation film, and the like are occurred.

To suppress the heat generation, the foregoing measure for releasingheat is taken. However, there is a problem in any method, such that themeasure for releasing heat is not sufficient, the backlight unit becomesthicker, or the like.

In view of the foregoing, it is an object of the present invention tomanufacture a highly reliable backlight device with less colorunevenness and less luminance unevenness, and a high-performance andhighly reliable display device including the backlight device, which candisplay high quality image.

In the present invention, a light emitting diode (hereinafter, alsoreferred to as LED) is used as a light source of a backlight device(also referred, to as a backlight or a lighting device) andthermoelectric elements are provided in a chassis for holding the lightemitting diode so that the thermoelectric elements surround the lightemitting diode (the thermoelectric elements are provided under the lightemitting diode and on the four sides thereof). A temperature in thebacklight device is adjusted by cooling and heating by thethermoelectric elements. The thermoelectric element refers to an elementof metal or semiconductor which converts heat energy to and fromelectric energy with use of a phenomenon relating heat and electricity.As an example of the thermoelectric element which can be used in thepresent invention, a Peltier element can be given.

When light is emitted from the light emitting diode, the temperature inthe backlight device changes. A temperature sensor is provided in thebacklight device to monitor a temperature condition, the thermoelectricelement carries out cooling or heating by a driving circuit for drivingthe thermoelectric element, and the temperature is controlled with atemperature controller. Further, a color sensor for monitoring output ofthe light emitting diode is provided, and the output of the lightemitting diode is controlled by a light emitting diode controllingdevice for controlling the output of the light emitting diode. The lightemitting diode is driven by a light emitting diode driving circuit.

A transmissive liquid crystal display panel module, which is provided infront of the backlight device, may also include a thermoelectric elementfor heating and cooling the liquid crystal display panel, a drivingcircuit for driving the thermoelectric element, a temperature sensor formonitoring a temperature condition of the (color) liquid crystal displaypanel, and a temperature controller for controlling the temperature.

A temperature sensor and a temperature controller may be provided foreach of the thermoelectric element provided in the backlight device andthe thermoelectric element provided in the liquid crystal display panelmodule so that the thermoelectric elements can be independentlyoperated. Alternatively, the temperature sensor and the temperaturecontroller may be provided to be shared so that the thermoelectricelements are operated together.

The backlight device and the liquid crystal display panel module may belocated to be in contact with each other or to have a gap therebetween.In a case where the liquid crystal display panel module and thebacklight device are provided to be in contact with each other and thethermoelectric element provided in the backlight device is in contactwith the liquid crystal display panel module, a temperature of theliquid crystal display panel module can be also controlled by coolingand heating by the thermoelectric element.

In addition, a thermoelectric module may be provided in a liquid crystaldisplay device to utilize temperature variation in the liquid crystaldisplay device for driving another light emitting diode, athermoelectric element, and the like. Since a thermoelectric elementwhich can carry out cooling and heating efficiently is included in achassis in the present invention, desired temperature variation iseasily obtained in the liquid crystal display device.

A mode of a backlight device of the present invention includes aplurality of light emitting diodes located in a chassis having athermoelectric element, in which the chassis having the thermoelectricelement is provided so as to surround the plurality of light emittingdiodes.

A mode of a backlight device of the present invention includes a firstlight emitting diode, a second light emitting diode, and a third lightemitting diode located in a chassis having a thermoelectric element; inwhich the chassis having the thermoelectric element is provided so as tosurround the first, the second, and the third light emitting diodes; anemission color of the first light emitting diode is red; an emissioncolor of the second light emitting diode is green; and an emission colorof the third light emitting diode is blue.

A mode of a backlight device of the present invention includes a firstlight emitting diode, a second light emitting diode, and a third lightemitting diode located in a chassis having a thermoelectric element; inwhich the chassis having the thermoelectric element is provided so as tosurround the first, the second, and the third light emitting diodes; thefirst light emitting diode has a peak in wavelength of an emission colorat 625 nm±10 nm; the second light emitting diode has a peak inwavelength of an emission color at 530 nm±15 nm; and the third lightemitting diode has a peak in wavelength of an emission color at 455nm±10 nm.

A mode of a display device of the present invention includes a backlight device having a plurality of light emitting diodes located in achassis having a thermoelectric element, and a display module; in whichthe chassis having the thermoelectric element is provided so as tosurround the plurality of light emitting diodes.

A mode of a display device of the present invention includes a backlight device having a plurality of light emitting diodes located in achassis having a Peltier element, and a display module; in which thechassis having the Peltier element is provided so as to surround theplurality of light emitting diodes.

A mode of a display device of the present invention includes a backlight device having a plurality of light emitting diodes located in achassis having a first thermoelectric element, and a display modulehaving a second thermoelectric element; in which the chassis having thefirst thermoelectric element is provided so as to surround the pluralityof light emitting diodes.

A mode of a display device of the present invention includes a backlight device having a plurality of light emitting diodes located in achassis having a first Peltier element, and a display module having asecond Peltier element; in which the chassis having the first Peltierelement is provided so as to surround the plurality of light emittingdiodes.

As a plurality of light emitting diodes used in the present invention,light emitting diodes which emit light of different colors can be used.For example, a red light emitting diode, a green light emitting diode,and a blue light emitting diode can be included. In specific, aplurality of light emitting diodes can include a first light emittingdiode having a peak in wavelength of an emission color at 625 nm±10 nm,a second light emitting diode having a peak in wavelength of an emissioncolor at 530 nm±15 nm, and a third light emitting diode having a peak inwavelength of an emission color at 455 nm±10 nm.

With the present invention, heat generation of the light emitting diodeused in the light source can be suppressed; therefore, reduction in lifetime and luminance, and chromaticity shift of the light emitting diodecan be suppressed. Since heat generation in the light source issuppressed, transformation and alteration of a diffusing film, areflective film, and a prism film can also be suppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Since light emitting diodes of three primary colors of light are used,color temperature can be easily adjusted and also, a colorreproducibility range can be widened compared with a cold cathodefluorescent lamp. Further, with use of the light emitting diode, atemperature range in which the display device can be used becomes widerand a moving picture can be easily displayed since a response speedbecomes high. Also, driving by low voltage becomes possible, an inverteris not required, a contrast can be improved, and mercury is not used,which is good for the environment.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views showing adisplay device of the present invention;

FIGS. 2A to 2C are a top view and cross-sectional views showing adisplay device of the present invention;

FIGS. 3A to 3C are a top view and cross-sectional views showing adisplay device of the present invention;

FIGS. 4A to 4C are a top view and cross-sectional views showing adisplay device of the present invention;

FIGS. 5A to 5C are a top view and cross-sectional views showing adisplay device of the present invention;

FIG. 6 shows a display device of the present invention;

FIG. 7 is a block diagram showing a display device of the presentinvention;

FIG. 8 is a block diagram showing a display device of the presentinvention;

FIG. 9 is a block diagram showing a display device of the presentinvention;

FIGS. 10A and 10B are graphs showing experimental data of Embodiment 1;

FIGS. 11A to 11C are graphs showing experimental data of Embodiment 2;

FIGS. 12A and 12B are graphs showing experimental data of Embodiment 1;

FIG. 13 is a graph showing experimental data of Embodiment 1;

FIG. 14 is a cross-sectional view showing a display device of thepresent invention;

FIG. 15 is a cross-sectional view showing a display device of thepresent invention;

FIGS. 16A to 16C are top views showing a display device of the presentinvention;

FIGS. 17A and 17B are top views showing a display device of the presentinvention;

FIGS. 18A to 18D are cross-sectional views showing a light emittingdiode which can be applied to the present invention;

FIGS. 19A to 19C are block diagrams showing a display device of thepresent invention;

FIG. 20 is a block diagram showing a main structure of an electronicappliance to which the present invention can be applied;

FIGS. 21A to 21C show electronic appliances of the present invention;

FIGS. 22A to 22E show electronic appliances of the present invention;

FIG. 23 is a top view of a display device of the present invention;

FIGS. 24A to 24C are a top view and cross-sectional views showing adisplay device of the present invention;

FIGS. 25A to 25C are cross-sectional views showing a display device ofthe present invention;

FIGS. 26A to 26C are graphs showing experimental data of Embodiment 3;

FIGS. 27A and 27B show experimental condition of Embodiment 3; and

FIGS. 28A and 28B show experimental condition of Embodiment 3.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention are describedwith reference to the accompanying drawings. The present invention canbe carried out in many different modes, and it is easily understood bythose skilled in the art that modes and details can be modified invarious ways without departing from the purpose and the scope of thepresent invention. Accordingly, the present invention should not beinterpreted as being limited to the description of the embodiment modesto be given below. Note that like portions in the drawings fordescribing embodiment modes are denoted by the like reference numeralsand repeated explanations thereof are omitted.

Embodiment Mode 1

In this embodiment mode, a concept of a backlight device (also referredto as a backlight or a lighting device) and a display device providedwith the backlight. device of the present invention is described.

In the present invention, a light emitting diode (LED) is used as alight source of a backlight device and thermoelectric elements areprovided in a chassis for holding the light emitting diode so that thethermoelectric elements surround the light emitting diode (under thelight emitting diode and on the four sides thereof). A temperature inthe backlight device is adjusted by cooling and heating by thethermoelectric elements. The thermoelectric element refers to an elementof metal or semiconductor which converts heat energy to and fromelectric energy with use of a phenomenon relating heat and electricity.As an example of a thermoelectric element which can be used in thepresent invention, a Peltier element can be given.

FIG. 1A is a top view of a display device which is provided with abacklight unit having a thermoelectric element which uses a lightemitting diode (hereinafter, also referred to as LED) as a light source.FIGS. 1B and 1C are cross-sectional views taking along a line A-B inFIG. 1A. FIG. 1B is a schematic diagram which is simplified and FIG. 1Cshows the example in detail. A display device of the present inventionis a liquid crystal display device including a liquid crystal displayelement Since liquid crystal does not emit light by itself, a backlightunit is located on the back of a liquid crystal display panel 150 as alight source.

In FIGS. 1A to 1C, light emitting diodes 101 a to 101 c are included ina chassis 100 having a thermoelectric element. The backlight unitincludes the light emitting diode 101 a which is a red light emittingdiode, having a peak at 625 nm±10 nm, the light emitting diode 101 bwhich is a green light emitting diode having a peak at 530 nm±15 nm, andthe light emitting diode 101 c which is a blue light emitting diodehaving a peak at 455 nm±10 nm. Light emitted from the light emittingdiodes 101 a to 101 c passes through the liquid crystal display panel150 in a direction shown by arrows in FIGS. 1B and 1C and is emitted toa viewing side. The chassis 100 having a thermoelectric element of thepresent invention is provided so as to surround at least three sides ofthe light emitting diodes 101 a to 101 c like a box. With the liquidcrystal display panel 150 located over the chassis 100 having athermoelectric element, the light emitting diodes 101 a to 101 c areprovided in a box-shaped closed space formed by the chassis 100 having athermoelectric element and the liquid crystal display panel 150.

As shown in FIG. 1C, the reflective sheet 102 is provided so as to coverthe chassis 100 having a thermoelectric element and reflects light fromthe light emitting diodes 101 a to 101 c. The light guide plate 103 islocated just over the light emitting diodes 101 a to 101 c and thediffusing plate 104 and the prism sheet 105 are located thereover with acertain distance therebetween.

The light emitting diodes 101 a to 101 c are controlled to have acertain temperature by the chassis 100 having a thermoelectric elementhaving function of heating and cooling. The chassis 100 having athermoelectric element of the present invention is provided so as tosurround four sides of the light emitting diodes 101 a to 101 c.Accordingly, not only the vicinity of the light emitting diodes 101 a to101 c, but also the whole backlight unit can be cooled or heatedefficiently; therefore, a temperature can be accurately controlled withless unevenness.

When light is emitted from the light emitting diode, the temperature inthe backlight device changes. A temperature sensor is provided in thebacklight device to monitor a temperature condition, the thermoelectricelement carries out cooling or heating by a driving circuit for drivingthe thermoelectric element, and the temperature is controlled with atemperature controller. Further, a color sensor for monitoring output ofthe light emitting diode is provided, and the output of the lightemitting diode is controlled by a light emitting diode controllingdevice for controlling the output of the light emitting diode. The lightemitting diode is driven by a light emitting diode driving circuit. Asthe color sensor and the temperature sensor, a color sensor and atemperature sensor including an IC chip can be used. At this time, thecolor sensor and the temperature sensor may be preferably located sothat they can be hidden in a chassis portion of the display device.

A transmissive liquid crystal display panel module, which is provided infront of the backlight device, may also include a thermoelectric elementfor heating and cooling the liquid crystal display panel, a drivingcircuit for driving the thermoelectric element, a temperature sensor formonitoring a temperature condition of the (color) liquid crystal displaypanel, and a temperature controller for controlling the temperature.

A temperature sensor and a temperature controller may be provided foreach of the thermoelectric element provided in the backlight device andthe thermoelectric element provided in the liquid crystal display panelmodule so that the thermoelectric elements can be independentlyoperated. Alternatively, the temperature sensor and the temperaturecontroller may be provided to be shared so that the thermoelectricelements are operated together.

The backlight device and the liquid crystal display panel module may belocated to be in contact with each other or to have a gap therebetween.In a case where the liquid crystal display panel module and thebacklight device are provided to be in contact with each other and thethermoelectric element provided in the backlight device is in contactwith the liquid crystal display panel module, a temperature of theliquid crystal display panel module can be also controlled by coolingand heating by the thermoelectric element.

In addition, a thermoelectric module may be provided in the liquidcrystal display device to utilize temperature variation in the liquidcrystal display device for driving another light emitting diode, athermoelectric element, and the like. Since a thermoelectric elementwhich can carry out cooling and heating efficiently is included in thechassis in the present invention, desired temperature variation iseasily obtained in the liquid crystal display device.

The backlight unit can include various colors of light emitting diode(LED), such as red, green, blue, and white, as a light source. Whenvarious colors of light emitting diodes (LEDs) are used, colorreproducibility can be improved. Further, in a case where light emittingdiodes (LEDs) of RGB are used as a light source, the number andarrangement of the light emitting diodes of each color are notnecessarily the same. For example, the number of light emitting diodesof a color with low emission intensity (such as green) may be largerthan the number of light emitting diode of another color.

When a field sequential mode is applied in a case of using the lightemitting diodes of RGB, color display can be carried out by sequentiallylighting the light emitting diodes of RGB as time passes.

The light emitting diode is suitable for a large display device sinceluminance thereof is high. In addition, color reproducibility of thelight emitting diode is superior to that of a cold cathode fluorescenttube because color purity of each color of RGB is favorable. An area inwhich light emitting diodes are located can be reduced; therefore, anarrower frame can be realized when the light emitting diode is appliedto a small display device.

Further, for example, when a backlight device including a light emittingdiode is mounted on a large display device, the light emitting diode canbe located on the back of a substrate of the display device. In thiscase, the light emitting diodes can be arranged with predetermined gapstherebetween so that the colors thereof are sequentially arranged. Colorreproducibility can be improved due to the arrangement of the lightemitting diodes.

The backlight device including a light emitting diode is particularlysuitable for a large display device, and a high quality image can beprovided even in a dark place by enhancing a contrast ratio of the largedisplay device.

FIGS. 2A to 2C show a chassis having a thermoelectric element with adifferent shape. As shown in FIGS. 2A to 2C, a part of the chassishaving a thermoelectric element may be provided on the display panelside to which light is emitted from the backlight unit. When a chassis120 having a thermoelectric element is provided so as to surround thebacklight device, the whole backlight unit can be cooled or heated moreefficiently; therefore, a temperature can be accurately controlled withless unevenness.

Further, the backlight unit and a display panel may be provided incontact with each other in order to thin the display device. In adisplay device shown in FIGS. 24A to 24C, a chassis having athermoelectric element provided in the backlight unit is partially incontact with the display panel. When the backlight device and thedisplay panel are provided in contact with each other (or they areprovided extremely close to each other and partially in contact witheach other), the display panel is easily influenced by change intemperature in the backlight device. When the thermoelectric elementincluded in the chassis is provided in contact with the display panel,the display panel side is also cooled or heated, and the temperaturethereof can be controlled.

FIGS. 3A to 4C show the chassis having a thermoelectric element withdifferent shapes. FIGS. 3A to 4C show examples of chassis 130 and 140having a thermoelectric element provided in contact with both thebacklight unit and the display panel. FIGS. 4A to 4C show a structure inwhich the chassis 140 having a thermoelectric element covers a part of aviewing side, as shown in FIGS. 2A to 2C, and the chassis wraps thebacklight unit and the display panel. As shown in FIGS. 3A to 4C, thechassis having a thermoelectric element may cover the backlight unit andthe display panel continuously or the chassis having a thermoelectricelement may be provided separately in a backlight unit region and in adisplay panel region.

In addition, the light emitting diode used in the present invention maybe covered with a resin or the like. As shown in FIGS. 5A to 5C, adome-shaped (hemispherical) resin cover which diffuses light may beprovided around the light emitting diode. A shape of the resin cover isnot limited and may be a reverse taper shape.

In addition, a liquid crystal display panel module may be any one offollowing modes: a TN (Twisted Nematic) mode, an IPS(In-Plane-Switching) mode, a FFS (Fringe Field Switching) mode, a MVA(Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, anOCB (Optical Compensated Birefringence) mode, a FLC (FerroelectricLiquid Crystal) mode, or an AFLC (Anti Ferroelectric Liquid Crystal)mode.

With the present invention, heat generation of the light emitting diodeused in the light source can be suppressed; therefore, reduction in lifetime and luminance, and chromaticity shift of the light emitting diodecan be suppressed. Since heat generation in the light source issuppressed, transformation and alteration of a diffusing film, areflective film, and a prism film can also be suppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, a.retardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

Embodiment Mode 2

In this embodiment mode, a backlight device using the present inventionand a display device including the backlight device are described indetail.

A display device shown in FIG. 6 includes a backlight unit 230 includinga light emitting diode 201 (a light emitting element 201 a, a cover 201b, and a terminal 201 c), a thermal conductive layer 202 serving as athermal conductive adhesive agent, a metal core substrate 203, a chassis200 having a thermoelectric element, a reflective sheet 204, a lightguide plate 205, a diffusing plate 206, prism sheets 207 a and 207 b,and a color sensor 220, and also includes a liquid crystal display panel231 including a substrate 211, a layer 212 including a display element,a substrate 213, polarizing plates 214 and 210, and a liquid crystaldisplay panel driving circuit 232. The backlight unit 230 furtherincludes a temperature sensor 221 in a backlight unit region, atemperature controller 222, a thermoelectric element driving circuit223, a light emitting diode controlling device 224, and a light emittingdiode driving circuit 225.

A metal core substrate is a substrate in which a metal core is providedin an intermediate layer of the substrate and has characteristics suchas thermal uniformity, improved mechanical strength, a shieldingproperty, and the like. Since the metal core substrate has favorableheat conduction, it is easy to have a structure in which heat isconcentrated on the metal core and is released at a time.

Although the light emitting diode shown in FIG. 6 has the dome-shapedcover 201 b, a shape of the light emitting diode is not limited thereto.It may be a reverse-tapered shape. Alternatively, a plurality of lightemitting diodes may be covered with a resin. FIGS. 18A to 18D show anexample of a light emitting diode which can be applied to the presentinvention.

A light emitting diode in FIG. 18A has a structure in which a lightemitting element 400 is covered with a cover 401 including a fluorescentmaterial 402. The cover 401 has a reverse-tapered shape and a partitionwall may be provided around the cover 401. Various emission colors canbe obtained by a combination of emission colors from the fluorescentmaterial and the light emitting element. In addition, various kinds offluorescent materials may be used.

A light emitting diode in FIG. 18B has a structure in which a lightemitting element 410 is covered with a cover 411 having a projection anda depression on its surface. Since the cover 411 has a projection and adepression on its surface, light emitted from the light emitting element410 is diffused. A diffusing direction or a diffusing amount of lightcan be controlled with a shape of the projection and the depression.

A light emitting diode shown in FIG. 18C has a structure in which alight emitting element 420 is covered with a stacked layer includingcovers 422 and 421. The cover 422 may be provided for diffusing light ormay be provided as a protective film of the light emitting element.Thus, a stacked layer including a plurality of covers of the lightemitting element may be provided.

A light emitting diode shown in FIG. 18D has a structure in which lightemitting elements 430 a, 430 b, and 430 c are covered with a cover 431.A structure such that a plurality of light emitting elements are coveredwith one cover, may be employed. For example, a structure in which alight emitting element 430 a which is a red light emitting diode, alight emitting element 430 b which is a green light emitting diode, anda light emitting element 430 c which is a blue light emitting diode maybe covered with one cover may be employed. A cover portion of the lightemitting diode may be provided so as to surround the light emittingelement without being directly in contact, or the cover portion may bedirectly in contact with the light emitting element to form a resin orthe like as a film. In addition, as shown in FIG. 18A, the covers 411,421, 422, and 431 may include a fluorescent material or anotherdiffusing substance.

An example of an operation mechanism of a display device in thisembodiment mode is described with reference to a block diagram of FIG.7. A liquid crystal display device 250 includes a liquid crystal displaypanel module 241 including the liquid crystal display panel 231 and theliquid crystal display panel driving circuit 232; a backlight module 242including the backlight unit 230, the temperature sensor 221 in abacklight unit region, the temperature controller 222, thethermoelectric element driving circuit 223, the color sensor 220, thelight emitting diode controlling device 224, and a light emitting diodedriving circuit 225; and a thermoelectric module 226.

A light emitting diode in the backlight unit 230 emits light by thelight emitting diode driving circuit 225. The color sensor 220 providedin the backlight unit monitors light emitting diodes which emit light ofvarious specific colors (such as a red light emitting diode, a greenlight emitting diode, and a blue light emitting diode) to see ifpredetermined output from each light emitting diode is obtained.Feedback of information to the light emitting diode controlling device224 is carried out and the information is outputted to the lightemitting diode driving circuit 225.

In addition, a temperature of the backlight unit 230 is measured, by thetemperature sensor 221 provided in the backlight region, so that a stateof heat generation of the light emitting diode is monitored. When atemperature of the light emitting diode is out of (higher or lower than)a predetermined temperature range, the thermoelectric element is drivenby the thermoelectric element driving circuit 223 to cool or heat thelight emitting diode and the temperature is controlled by thetemperature controller 222, in order to prevent the light emitting diodefrom driving at outside of (higher or lower than) the temperature range.

Further, the thermoelectric module 226 is provided in the liquid crystaldisplay device so that temperature variation in the liquid crystaldisplay device can be utilized for driving of another light emittingdiode, a thermoelectric element, and the like. Since a thermoelectricelement which can carry out cooling and heating efficiently is includedin a chassis in the present invention, desired temperature variation iseasily obtained in the liquid crystal display device. The thermoelectricmodule 226 may be provided in any place in the liquid crystal displaydevice. The thermoelectric module 226 may be provided in a liquidcrystal display panel module region or in a backlight module region, oracross the both regions. Alternatively, the thermoelectric module 226may be provided separately from the liquid crystal display device. Aplurality of thermoelectric modules, color sensors, and temperaturesensors may be provided. When a plurality of color sensors andtemperature sensors are provided, monitoring can be carried out withfurther accuracy.

In addition, as shown in FIG. 8, a thermoelectric element may also beprovided in a liquid crystal display panel module 241. In this case, atemperature sensor 233, a temperature controller 234, and athermoelectric element driving circuit 235 are provided in a liquidcrystal display region. In addition, a temperature of the liquid crystaldisplay panel 231 is monitored by the temperature sensor 233 provided inthe liquid crystal display panel region. When a temperature of theliquid crystal display panel 231 is out of (higher or lower than) apredetermined temperature range, the thermoelectric element is driven bythe thermoelectric element driving circuit 235 provided in the liquidcrystal display panel 231 to cool or heat the liquid crystal displaypanel 231 and the temperature is controlled by the temperaturecontroller 234 so that the temperature of the liquid crystal displaypanel 231 is in the predetermined temperature range.

In FIG. 8, a structure in which the thermoelectric element drivingcircuits 235 and 223 are separately provided in the backlight unitregion and the liquid crystal display panel region, respectively. It ispossible that the thermoelectric element driving circuits operate inconjunction with each other. Alternatively, it is possible that theyoperate differently at the same time such that the thermoelectricelement driving circuit 235 operates to heat the thermoelectric elementand the thermoelectric element driving circuit 233 operates to cool thethermoelectric element.

FIG. 9 shows an example in which a thermoelectric element in the liquidcrystal display panel module 241 and a thermoelectric element in thebacklight module 242 are driven by one common thermoelectric elementdriving circuit 243. As shown in FIG. 9, when the thermoelectric elementdriving circuit is provided to be shared, there is an advantage suchthat the liquid crystal display device can be thinned and cost can bereduced.

With the present invention, heat generation of the light emitting diodeused in a light source can be suppressed; therefore, reduction in lifetime and luminance, and chromaticity shift of the light emitting diodecan be suppressed. Since heat generation in the light source issuppressed, transformation and alteration of a diffusing film, areflective film, and a prism film can also be suppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

Embodiment Mode 3

In this embodiment mode, a liquid crystal display device including abacklight device of the present invention and including a thin filmtransistor having a crystalline semiconductor film is described.

FIG. 16A is a top view showing a structure of a display device of thepresent invention, in which a pixel portion 2701 including pixels 2702arranged in matrix, a scanning line input terminal 2703, and a signalline input terminal 2704 are formed over a substrate 2700 with aninsulating surface. The number of pixels provided may be determined inaccordance with various standards. For example, the number of pixels ofXGA for RGB full-color display may be 1024×768×3 (RGB), the number ofpixels of UXGA for RGB full-color display may be 1600×1200×3 (ROB), andthe number of pixels corresponding to a full-speck high vision for ROBfull-color display may be 1920×1080×3 (RGB).

The pixels 2702 are arranged in matrix provided at intersections ofscanning lines extended from the scanning line input terminal 2703 andsignal lines extended from the signal line side input terminal 2704.Each pixel 2702 includes a switching element and a pixel electrode layerconnected thereto. As a representative example of a switching element, aTFT can be given. A TFT has a gate electrode layer side connected to thescanning line and a source or drain side connected to the signal line,so that each pixel can be controlled independently by a signal inputtedfrom an external portion.

Although FIG. 16A shows a structure of a display panel in which a signalinputted to the scanning line and the signal line is controlled by anexternal driving circuit, a driver IC 2751 may be mounted over asubstrate 2700 by a COG (Chip On Glass) method as shown in FIG. 17A.Alternatively, a TAB (Tape Automated Bonding) method shown in FIG. 17Bmay be employed. The driver IC may be one formed over a singlecrystalline semiconductor substrate or may be a circuit that is formedusing a TFT over a glass substrate. In FIGS. 17A and 17B, the driver IC2751 is connected to an FPC (Flexible Printed Circuit) 2750.

In a case of forming a TFT provided in a pixel with a semiconductor withcrystallinity, a scanning line driving circuit 3702 can also be formedover a substrate 3700, as shown in FIG. 16B. In FIG. 16B, a pixelportion 3701 is controlled by an external driving circuit connected to asignal line input terminal 3704, similarly to FIG. 16A. In a case wherea TFT provided in a pixel is formed of a polycrystalline(microcrystalline) semiconductor, a signal crystalline semiconductor, orthe like with high mobility, a pixel portion 4701, a scanning linedriving circuit 4702, and a signal line driving circuit 4704 can befowled over a substrate 4700.

FIG. 23 shows a top view of a liquid crystal display device including abacklight unit of the present invention and FIG. 14 is a cross-sectionalview taken along a line C-D in FIG. 23.

As shown in FIGS. 23 and 14, a pixel region 606, a driving circuitregion 608 a which is a scanning line driving circuit, and a drivingcircuit region 608 b which is a scanning line driving region are sealedwith a sealant 692 between a substrate 600 and a counter substrate 695.A driving circuit region 607 which is a signal line driving circuitformed with an IC driver is provided over the substrate 600. Atransistor 622 and a capacitor 623 are provided in the pixel region 606.A driving circuit having transistors 620 and 621 is provided in thedriving circuit region 608 b.

The substrate 600 and the counter substrate 695 are insulatingsubstrates with a light-transmitting property (hereinafter, alsoreferred to as a light-transmitting substrate). The substrateparticularly transmits light in a wavelength region of visible light.For example, a glass substrate such as a barium borosilicate glass oralumino borosilicate glass, or a quartz substrate can be used.Alternatively, a substrate formed from plastics represented bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyethersulfone (PES), and polycarbonate (PC); or a substrate formedfrom a synthetic resin with flexibility such as acrylic can be employed.Further alternatively, a film (formed from polypropylene, polyester,vinyl, polyvinyl fluoride, or vinyl chloride), a base film (formed frompolyester, polyamide, or an inorganic evaporated film), and the like maybe used. Although there is a concern that a substrate fowled from asynthetic resin generally has a low heat-resistance temperature comparedto another substrate, the substrate formed from a synthetic resin can beused when a manufacturing process is carried out with a substrate withhigh heat resistance and then the substrate formed from a syntheticresin displaces the substrate with high heat resistance.

In the pixel region 606, a transistor 622 which is to be a switchingelement is provided over the substrate. 600, with base films 604 a and604 b therebetween. In this embodiment mode, a multi-gate thin filmtransistor (TFT) is used as the transistor 622, which includes asemiconductor layer having an impurity region serving as a source regionand a drain region, a gate insulting layer, a gate electrode layerhaving a stacked-layer structure including two layers, a sourceelectrode layer and a drain electrode layer. The source electrode layeror the drain electrode layer is in contact with and electricallyconnected to an impurity region in the semiconductor layer and a pixelelectrode layer 630. The thin film transistor can be manufactured byvarious methods. For example, a crystalline semiconductor film is usedas an active layer, a gate electrode is formed thereover with a gateinsulating film therebetween, and an impurity element is added to theactive layer with use of the gate electrode. In such a manner, when thegate electrode is used for adding the impurity element, a mask foradding the impurity element is not required. The gate electrode can havea single layer structure or a stacked-layer structure. The impurityregion can be a high concentration impurity region or a lowconcentration impurity region with its concentration being controlled. Astructure of a thin film transistor having a low impurity region iscalled an LDD (Light Doped Drain) structure. Alternatively, the lowconcentration impurity region may be overlapped with the gate electrodeand a structure of such a thin film transistor is called a GOLD (GateOverlapped LDD) structure. Polarity of the thin film transistor becomesan n-type when phosphorus (P) or the like is added to the impurityregion. The polarity of the thin film transistor becomes a p-type whenboron (B) or the like is added. After that, insulating films 611 and 612covering the gate electrode and the like are formed. A dangling bond ofthe crystalline semiconductor film can be terminated by a hydrogenelement mixed into the insulating film 611 (and the insulating 612).

In order to further improve planarity, insulating films 615 and 616 maybe formed as interlayer insulating films. For the insulating films 651and 616, an organic material, an inorganic material, or a stacked-layerstructure thereof can be used. For example, a material selected fromsilicon oxide, silicon nitride, silicon oxynitride, silicon nitrideoxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxidecontaining more nitrogen than oxygen, aluminum oxide, diamond likecarbon (DLC), polysilazane, nitrogen-containing carbon (CN), PSG(phosphosilicate glass), or BPSG (borophosphosilicate glass), alumina,and a substance containing another inorganic insulating material can beused. Alternatively, an organic insulating material 1 may be used. Asthe organic material, either a photosensitive or nonphotosensitivematerial can be used, and polyimide, acryl, polyamide, polyimide amid;resist, benzocyclobutene, a siloxane resin, or the like can be used.Note that a siloxane resin is a resin including a Si—O—Si bond. Theskeletal structure of siloxane is formed by a bond of silicon (Si) andoxygen (O). As a substituent, an organic group including at leasthydrogen (such as an alkyl group or an aromatic hydrocarbon) is used. Asthe substituent, a fluoro group may be used. Alternatively, an organicgroup containing at least hydrogen, and a fluoro group may be used asthe substituent.

The pixel region and the driving circuit region can be formed over onesubstrate when the crystalline semiconductor film is used. In this case,a transistor in the pixel portion and a transistor in the drivingcircuit region 608 b are fainted at the same time. The transistor usedin the driving circuit region 608 b is included in a CMOS circuit. Athin film transistor included in the CMOS circuit has a GOLD structure,but it may have an LDD structure like the transistor 622.

A structure of the thin film transistor in the pixel region is notlimited to those referred to in this embodiment mode and the thin filmtransistor in the pixel region may have a single gate structure with onechannel formation region, a double gate structure with two channelformation regions, or a triple gate structure with three channelformation regions. A thin film transistor in a peripheral drivingcircuit region may also have a single gate structure, a double gatestructure, or a triple gate structure.

Note that a manufacturing method of a thin film transistor is notlimited to those referred to in this embodiment mode. The thin filmtransistor may have a top gate structure (such as a staggered type), abottom gate structure (such as an inversed staggered type), a dual gatestructure in which two gate electrode layers are arranged at above orbelow a channel formation region, each with a gate insulating filminterposed therebetween, or another structure.

Then, an insulating layer 631 referred to as an orientation film isformed by a printing method or a droplet discharge method, to cover thepixel electrode layer 630 and the insulating film 616. Note that theinsulating layer 631 can be selectively formed by a screen printingmethod or an off-set printing method. Thereafter, a rubbing treatment isperformed. This rubbing treatment is not necessarily performed when aliquid crystal mode is, for example, a VA mode. An insulating layer 633serving as an orientation film is similar to the insulating layer 633.Subsequently, the sealant 692 is formed in a region surrounding thepixels by a droplet discharge method.

After that, the counter substrate 695 provided with the insulating layer633 serving as the orientation film, a conductive layer 634 serving as acounter electrode, and a color layer 635 serving as a color filter isattached to the substrate 600 which is a TFT substrate, with a spacer637 therebetween. A liquid crystal layer 632 is provided in a spacetherebetween. Then, a polarizing plate 641 is provided on an outer sideof the counter substrate 695 and a polarizing plate 643 is provided on aside of the substrate 600 which is opposite to a side where an elementis formed. The polarizing plate can be provided on the substrate withuse of an adhesive layer. In addition, a retardation plate may beprovided between the polarizing plate and the substrate. A filler may bemixed into the sealant, and a shielding film (black matrix) or the likemay be formed on the counter substrate 695. Note that a color filter orthe like may be formed of materials which exhibit red (R), green (G),and blue (B) when the liquid crystal display device is a full-colordisplay; and the color layer may be omitted or may be formed of amaterial which exhibits at least one color, when the liquid crystaldisplay device is a single-color display.

Note that when RGB light emitting diodes (LEDs) or the like are locatedin the backlight device and a field sequential method which conductscolor display by time division is employed, there is a case where acolor filter is not provided. The black matrix may be provided so as tooverlap with the transistor and the CMOS circuit since the black matrixreduces the reflection of outside light by the wiring in the transistorand the CMOS circuit. Alternatively, the black matrix may be provided tooverlap with the capacitor. It is because the black matrix can preventreflection due to a metal film included in the capacitor.

As a method for forming the liquid crystal layer, a dispenser method(dripping method) or an injection method in which the substrate 600provided with an element and the counter substrate 695 are attached andliquid crystal is injected with use of capillary phenomenon can be used.A dripping method may be employed when a large substrate to which aninjection method is difficult to be applied is used.

A spacer may be provided by a method in which particles each having asize of several micrometers are sprayed. In this embodiment mode, amethod in which a resin film is formed over the entire surface of thesubstrate and then etched is employed. A material for the spacer isapplied by a spinner and then, light exposure and a developing treatmentare carried out so as to form a predetermined pattern. Further, thepattern is heated at 150 to 200° C. in a clean oven or the like to behardened. The spacer manufactured in such a manner can have variousshapes depending on the conditions of light exposure and the developingtreatment. It is preferable that the spacer have a columnar shape with aflat top so that mechanical strength of the liquid crystal displaydevice can be secured when the counter substrate is attached. The shapeof the spacer is not particularly limited and may be conic, pyramidal,or the like.

Then, an FPC 694, which is a wiring board for connection is providedover a terminal electrode layer 678 electrically connected to the pixelregion, with an anisotropic conductive layer 696 therebetween. The FPC694 transmits a signal and potential from an external portion. Throughthe foregoing steps, a liquid crystal display device having a displayfunction can be manufactured.

For the wiring in the transistor, the gate electrode layer, the pixelelectrode layer 630, and the conductive layer 634 serving as the counterelectrode can be formed with use of indium tin oxide (ITO), indium zincoxide (IZO) in which zinc oxide (ZnO) is mixed with indium oxide, aconductive material in which silicon oxide (SiO₂) is mixed with indiumoxide, organic indium, organic tin, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,or the like. Alternatively, a material selected from a metal such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), orsilver (Ag); an alloy of such metals; or metal nitride thereof can beused.

The backlight unit provided in the display device shown in FIG. 14includes a light emitting diode 651, a chassis 650 having athermoelectric element, a reflective sheet 652, a light guide plate 653,a diffusing plate 654, a prism sheet 655, and a color sensor 656.

In this embodiment mode, a light emitting diode is used as a lightsource of a backlight device and thermoelectric elements are provided ina chassis for holding the light emitting diode so that thethermoelectric elements surround the light emitting diode (under thelight emitting diode and on the four sides thereof). A temperature inthe backlight device is adjusted by cooling and heating by thethermoelectric elements. A thermoelectric element refers to an elementof metal or semiconductor which converts heat energy to and fromelectric energy with use of a phenomenon relating heat and electricity.As an example of a thermoelectric element which can be used in thepresent invention, a Peltier element can be given.

When light is emitted from the light emitting diode, the temperature inthe backlight device changes. A temperature sensor is provided in thebacklight device to monitor a temperature condition, the thermoelectricelement carries out cooling or heating by a driving circuit for drivingthe thermoelectric element, and the temperature is controlled with atemperature controller in the following manner. Further, a color sensorfor monitoring output of the light emitting diode is provided, and theoutput of the light emitting diode is controlled by a light emittingdiode controlling device for controlling the output of the lightemitting diode. The light emitting diode is driven by a light emittingdiode driving circuit.

A transmissive liquid crystal display panel module, which is provided infront of the backlight device, may also include a thermoelectric elementfor heating and cooling the liquid crystal display panel, a drivingcircuit for driving the thermoelectric element, a temperature sensor formonitoring a temperature condition of the (color) liquid crystal displaypanel, and a temperature controller for controlling the temperature.

A temperature sensor and a temperature controller may be provided foreach of the thermoelectric element provided in the backlight device andthe thermoelectric element provided in the liquid crystal display panelmodule so that the thermoelectric elements can be independentlyoperated. Alternatively, the temperature sensor and the temperaturecontroller may be provided to be shared so that the thermoelectricelements are operated together.

The backlight device and the liquid crystal display panel module may belocated to be in contact with each other or to have a gap therebetween.In a case where the liquid crystal display panel module and thebacklight device are provided to be in contact with each other and thethermoelectric element provided in the backlight device is in contactwith the liquid crystal display panel module, a temperature of theliquid crystal display panel module can be also controlled by coolingand heating by the thermoelectric element.

In addition, a thermoelectric module may be provided in the liquidcrystal display device to utilize temperature variation in the liquidcrystal display device for driving another light emitting diode, athermoelectric element, and the like. Since a thermoelectric elementwhich can carry out cooling and heating efficiently is included in thechassis in the present invention, desired temperature variation iseasily obtained in the liquid crystal display device.

With the present invention, heat generation of the light emitting diodeused in the light source can be suppressed; therefore, reduction in lifetime and luminance, and chromaticity shift of the light emitting diodecan be suppressed. Since heat generation in the light source issuppressed, transformation and alteration of a diffusing film, areflective film, and a prism film can also be suppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

This embodiment mode can be freely combined with the foregoingembodiment modes.

Embodiment Mode 4

In this embodiment mode, a liquid crystal display device including abacklight device of the present invention and using a thin filmtransistor which has an amorphous semiconductor film is described.

The display device shown in FIG. 15 includes a transistor 320 which isan inversed staggered thin film transistor, a pixel electrode layer 301,an insulating layer 302, an insulating layer 303, a liquid crystal layer304, a spacer 381, an insulating layer 305, a counter electrode layer306, a color filter 308, a black matrix 307, a counter substrate 310, apolarizing plate 331, and a polarizing plate 333 in a pixel region; asealant 382, a terminal electrode layer 387, an anisotropic conductivelayer 385, and an FPC 386 in a sealing region; over a substrate 300.

The transistor 320 which is an inversed staggered thin film transistorformed in this embodiment mode has a gate electrode layer, a sourceelectrode layer, and a drain electrode layer formed by a dropletdischarge method. The droplet discharge method is a method in which aconductive layer and an electrode layer are formed by discharging acomposition containing a liquid conductive material and solidifying thecomposition. by drying and baking. If a composition containing aninsulating material is discharged and solidified through drying andbaking, an insulating layer can also be formed. When a droplet dischargemethod is employed, a constituent of a display device, such as aconductive layer and an insulating layer can be selectively formed,which can simplify the process and reduce a waste of materials;therefore, a display device can be manufactured at low cost and withhigh productivity.

In this embodiment mode, an amorphous semiconductor is used as asemiconductor layer, and a semiconductor layer having one conductivitymay be formed if needed. In this embodiment mode, a semiconductor layerand an n-type amorphous semiconductor layer which is the semiconductorlayer having one conductivity are stacked. in addition, an n-channelthin film transistor with an NMOS structure which includes an n-typesemiconductor layer, a p-channel thin film transistor with a PMOSstructure which includes a p-type semiconductor layer, or a CMOSstructure which includes an n-channel thin film transistor and ap-channel thin film transistor can be manufactured.

In addition, an n-channel thin film transistor and a p-channel thin filmtransistor can be formed by adding an element imparting conductivity bydoping and forming an impurity region in a semiconductor layer. Further,instead of formation of the n-type semiconductor layer, a plasmatreatment may be performed with a PH₃ gas to impart conductivity to thesemiconductor layer.

In this embodiment mode, the transistor 320 is an n-channel inversedstaggered thin film transistor. Alternatively, an inversed staggeredthin film transistor of a channel protective type in which a protectivelayer is provided over a channel region in the semiconductor layer maybe used.

The semiconductor layer can be formed with use of an organicsemiconductor material as a semiconductor, by a vapor deposition method,a printing method, a spray method, a spin coating method, a dropletdischarge method, a dispenser method, or the like. In this case, sincean etching step is not always necessary, the number of steps can bereduced. As an organic semiconductor, a low molecular material such aspentacene or a high molecular material can be used, or a material suchas an organic pigment or a conductive high molecular organic materialcan be used. As an organic semiconductor material used in the presentinvention, a π-conjugated high molecular material with its skeletonincluding a conjugate double bond is desirable. Typically, a highmolecular material which is soluble in a liquid such as polythiophene,polyfluorene, poly(3-alkylthiophene), or a polythiophene derivative canbe used.

A backlight unit included in the display device shown in FIG. 15includes a light emitting diode 351, a chassis 350 having athermoelectric element, a reflective sheet 352, a light guide plate 353,a diffusing plate 354, a prism sheet 355, and a color sensor 356.

In this embodiment mode, a light emitting diode is used as a lightsource of a backlight device and thermoelectric elements are provided ina chassis for holding the light emitting diode so that thethermoelectric elements surround the light emitting diode (under thelight emitting diode and on the four sides thereof). A temperature inthe backlight device is adjusted by cooling and heating by thethermoelectric elements. A thermoelectric element refers to an elementof metal or semiconductor which converts heat energy to and fromelectric energy with use of a phenomenon relating heat and electricity.As an example of a thermoelectric element which can be used in thepresent invention, a Peltier element can be given.

When light is emitted from the light emitting diode, the temperature inthe backlight device changes. A temperature sensor is provided in thebacklight device to monitor a temperature condition, the thermoelectricelement carries out cooling or heating by being controlled by a drivingcircuit for driving the thermoelectric element, and the temperature iscontrolled with a temperature controller. Further, a color sensor formonitoring output of the light emitting diode is provided, and theoutput of the light emitting diode is controlled by a light emittingdiode controlling device for controlling the output of the lightemitting diode. The light emitting diode is driven by a light emittingdiode driving circuit.

A transmissive liquid crystal display panel module, which is provided infront of the backlight device, may also include a thermoelectric elementfor heating and cooling the liquid crystal display panel, a drivingcircuit for driving the thermoelectric element, a temperature sensor formonitoring a temperature condition of the (color) liquid crystal displaypanel, and a temperature controller for controlling the temperature.

A temperature sensor and a temperature controller may be provided foreach of the thermoelectric element provided in the backlight device andthe thermoelectric element provided in the liquid crystal display panelmodule so that the thermoelectric elements can be independentlyoperated. Alternatively, the temperature sensor and the temperaturecontroller may be provided to be shared so that the thermoelectricelements are operated at the same time.

The backlight device and the liquid crystal display panel module may belocated to be in contact with each other or to have a gap therebetween.In a case where the liquid crystal display panel module and thebacklight device are provided to be in contact with each other and thethermoelectric element provided in the backlight device is in contactwith the liquid crystal display panel module, a temperature of theliquid crystal display panel module can be also controlled by coolingand heating by the thermoelectric element.

In addition, a thermoelectric module may be provided in the liquidcrystal display device to utilize temperature variation in the liquidcrystal display device for driving another light emitting diode, athermoelectric element, and the like. Since a thermoelectric elementwhich can carry out cooling and heating efficiently is included in thechassis in the present invention, desired temperature variation iseasily obtained in the liquid crystal display device.

With the present invention, heat generation of the light emitting diodeused in the light source can be suppressed; therefore, reduction in lifetime and luminance, and chromaticity shift of the light emitting diodecan be suppressed. Since heat generation in the light source issuppressed, transformation and alteration of a diffusing film, areflective film, and a prism film can also be suppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

This embodiment mode can be freely combined with the foregoingembodiment modes.

Embodiment Mode 5

In this embodiment mode, another structural example of a backlightdevice of the present invention and of a display device including thebacklight device are described.

A backlight device of the present invention may be a sidelight type(also referred to as an edgelight type) as shown in FIGS. 25A and 25B.

A display device shown in FIG. 25A includes a liquid crystal displaypanel module 459, a chassis 450 having a thermoelectric element, lightemitting diodes 451 a and 451 b, reflective sheets 452 a, 452 b, and 452c, a diffusing plate 454, and a prism sheet 455. The light emittingdiode 451 a and 451 b are provided on lateral portions of the chassis450 having a thermoelectric element. The reflective sheets 452 a, 452 b,and 452 c reflect light emitted from the light emitting diodes 451 a and451 b to the liquid crystal display panel module side. The reflectivesheet 452 a has a projection in the center so that the light isreflected to be propagated in the backlight device efficiently.

A display device shown in FIG. 25B includes a liquid crystal displaypanel module 479, a chassis 470 having a thermoelectric element, lightemitting diodes 471 a and 471 b, a light guide plate 473, a diffusingplate 474, and a prism sheet 475. The light emitting diodes 471 a and471 b are provided on lateral portions of the chassis 470 having athermoelectric element. The light guide plate 473 propagates lightemitted from the Light emitting diodes 471 a and 471 b in the backlightdevice so that the light is emitted to the liquid crystal display panelmodule side.

A display device shown in FIG. 25C includes a liquid crystal displaypanel module 469, a chassis 460 having a thermoelectric element, a lightemitting diode 461, reflective sheets 462 a, 462 b, 462 c, and 462 d,light guide plates 463 a and 463 b, a diffusing plate 464, and a prismsheet 465. The light emitting diode 461 is provided on the bottomportion of the chassis 460 having a thermoelectric element. Thereflective sheets 462 a and 462 b reflect light emitted from the lightemitting diode 461 and propagate the light to the light guide plate 463a. The light propagated by the light guide plate 463 a is reflected bythe reflective sheet 462 c, propagated by the light guide plate 463 b,and then, emitted to the liquid crystal display panel module side by thereflective sheet 462 d.

As a plurality of light emitting diodes used in the present invention,light emitting diodes which emit light of different colors can be used.For example, a red light emitting diode, a green light emitting diode,and a blue light emitting diode can be included. In specific, aplurality of light emitting diodes can include a first light emittingdiode having a peak in wavelength of an emission color at 625 nm±10 nm,a second light emitting diode having a peak in wavelength of an emissioncolor at 530 nm±15 nm, and a third light emitting diode having a peak inwavelength of an emission color at 455 nm±10 nm.

In this embodiment mode, a light emitting diode is used as a lightsource of a backlight device and thermoelectric elements are provided ina chassis for holding the light emitting diode so that thethermoelectric elements surround the light emitting diode (under thelight emitting diode and on the four sides thereof). A temperature inthe backlight device is adjusted by cooling and heating by thethermoelectric elements. A thermoelectric element refers to an elementof metal or semiconductor which converts heat energy to and fromelectric energy with use of a phenomenon relating heat and electricity.As an example of a thermoelectric element which can be used in thepresent invention, a Peltier element can be given.

When light is emitted from the light emitting diode, the temperature inthe backlight device changes. A temperature sensor is provided in thebacklight device to monitor a temperature condition, the thermoelectricelement carries out cooling or heating by being controlled by a drivingcircuit for driving the thermoelectric element, and the temperature iscontrolled with a temperature controller. Further, a color sensor formonitoring output of the light emitting diode is provided, and theoutput of the light emitting diode is controlled by a light emittingdiode controlling device for controlling the output of the lightemitting diode. The light emitting diode is driven by a light emittingdiode driving circuit.

A transmissive liquid crystal display panel module, which is provided infront of the backlight device, may also include a thermoelectric elementfor heating and cooling the liquid crystal display panel, a drivingcircuit for driving the thermoelectric element, a temperature sensor formonitoring a temperature condition of the (color) liquid crystal displaypanel, and a temperature controller for controlling the temperature.

A temperature sensor and a temperature controller may be provided foreach of the thermoelectric elements provided in the backlight device andthe thermoelectric element provided in the liquid crystal display panelmodule so that the thermoelectric elements can be independentlyoperated. Alternatively, the temperature sensor and the temperaturecontroller may be provided to be shared so that the thermoelectricelements are operated together.

The backlight device and the liquid crystal display panel module may belocated to be in contact with each other or to have a gap therebetween.In a case where the liquid crystal display panel module and thebacklight device are provided to be in contact with each other and thethermoelectric element provided in the backlight device is in contactwith the liquid crystal display panel module, a temperature of theliquid crystal display panel module can be also controlled by coolingand heating by the thermoelectric element.

In addition, a thermoelectric module may be provided in the liquidcrystal display device to utilize temperature variation in the liquidcrystal display device for driving another light emitting diode, athermoelectric element, and the like. Since a thermoelectric elementwhich can carry out cooling and heating efficiently is included in thechassis in the present invention, desired temperature variation iseasily obtained in the liquid crystal display device.

With the present invention, heat generation of the light emitting diodeused in the light source can be suppressed; therefore, reduction in lifetime and luminance, and chromaticity shift of the light emitting diodecan be suppressed. Since heat generation in the light source issuppressed, transformation and alteration of a diffusing film, areflective film, and a prism film can also be suppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panel.can also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

This embodiment mode can be freely combined with the foregoingembodiment modes.

Embodiment Mode 6

In this embodiment mode, operation of each circuit or the like includedin a display device is described.

FIGS. 19A to 19C show system block diagrams of a pixel portion 505 and adriving circuit portion 508 in a display device.

In the pixel portion 505, a plurality of pixels are included. Switchingelements are provided in each intersection region of signal lines 512and scanning lines 510, which serves as a pixel. Application of voltagefor controlling tilt of liquid crystal molecules can be controlled bythe switching elements. Such a structure in which switching elements areprovided in each intersection region is called an active type. A pixelportion of the present invention is not limited to such an active type,and may have a passive type structure instead. The passive type ismanufactured by a simple process because a switching element is notincluded in each pixel.

The driving circuit portion 508 includes a controlling circuit 502, asignal line driving circuit 503, and a scanning line driving circuit504. The controlling circuit 502 to which an image signal 501 isinputted has a function to control a gray scale in accordance withcontents to be displayed of the pixel portion 505. The controllingcircuit 502 inputs a generated signal to the signal line driving circuit503 and the scanning line driving circuit 504. When a switching elementis selected by the scanning line driving circuit 504 with the scanningline 510, voltage is applied to a pixel electrode in a selectedintersection region. A value of this voltage is determined based on asignal inputted from the driving circuit 503 through the signal line.

Further, in the controlling circuit 502, a signal for controllingelectric power supplied to a lighting unit 506 is generated. The signalis inputted to a power supply 507 of the lighting unit 506. As thelighting unit, the backlight unit shown in the foregoing embodimentmodes can be used. Note that the lighting unit may be a frontlightinstead of the backlight device. A frontlight is a platy light unitwhich includes a light emitter which illuminates the whole liquidcrystal display panel module and a light guide body, and which isattached to a front surface side of the pixel portion. With such alighting unit, the pixel portion can be evenly irradiated with light,with low power consumption.

As shown in FIG. 19B, a scanning line driving circuit 504 includescircuits serving as a shift resister 541, a level shifter 542, and abuffer 543. Signals such as a gate start pulse (GSP) and a gate clocksignal (GCK) are inputted to the shift resister 541. Note that astructure of the scanning line driving circuit of the present inventionis not limited to the structure shown in FIG. 19B.

As shown in FIG. 19C, the signal line driving circuit 503 includescircuits serving as a shift resister 531, a first latch 532, a secondlatch 533, a level shifter 534, and a buffer 535. The circuit serving asthe buffer 535 is a circuit having a function for amplifying a weaksignal and includes an operational amplifier and the like. A signal suchas a start pulse (SSP) and data (DATA) such as a video signal areinputted to the level shifter 534 and the first latch 532, respectively.Latch (LAT) signals can be temporarily held in the second latch 533, andthey are inputted to the pixel portion 505 at a time. Such operation isreferred to as line sequential driving. If the pixel performs dotsequential driving instead of the line sequential driving, the secondlatch is not required. Thus, a structure of a signal driving circuit ofthe present invention is not limited to the structure shown in FIG. 19C.

The signal line driving circuit 503, the scanning line driving circuit504, and the pixel portion 505 as described above can be formed ofsemiconductor elements provided over one substrate. The semiconductorelement can be formed with use of a thin film transistor formed over aglass substrate. In this case, a crystalline semiconductor film may beapplied to the semiconductor elements (refer to Embodiment Mode 5). Acrystalline semiconductor film can be included in a circuit in a drivingcircuit portion since its characteristics, in particular mobility, ishigh. Further, the signal line driving circuit 503 and the scanning linedriving circuit 504 can be mounted over the substrate with use of an IC(Integrated Circuit) chip. In this case, an amorphous semiconductor filmcan be applied to a semiconductor element in the pixel portion (refer toEmbodiment Mode 5).

In such a display device, a light emitting diode is used as a lightsource of the backlight device and thermoelectric elements are providedin a chassis for holding the light emitting diode so that thethermoelectric elements surround the light emitting diode (under thelight emitting diode and on the four sides thereof). A temperature inthe backlight device is adjusted by cooling and heating by thethermoelectric elements. With the present invention, heat generation ofthe light emitting diode used in the light source can be suppressed;therefore, reduction in life time and luminance, and chromaticity shiftof the light emitting diode can be suppressed. Since heat generation inthe light source is suppressed, transformation and alteration of adiffusing film, a reflective film, and a prism film can also besuppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

Embodiment Mode 7

With use of a display device formed by the present invention, atelevision device (also simply referred to as a television or atelevision receiver) can be completed. FIG. 20 is a block diagram of amain structure of a television device. A display panel can employ anyone of the following structure: a structure shown in FIG. 16A in whichonly a pixel portion 701 is formed on the display panel and a scanningline driving circuit 703 and a signal line driving circuit 702 aremounted thereon by a TAB method shown in FIG. 17B, or by a COG methodshown in FIG. 17A; a structure shown in FIG. 16B in which a TFT isformed, the pixel portion 701 and the scanning line driving circuit 703are formed over a substrate, and the signal line driving circuit 702 isseparately mounted as a driver IC, or a structure shown in FIG. 17C inwhich the pixel portion 701, the signal line driving circuit 702, andthe scanning line driving circuit 703 are formed over the samesubstrate.

As another structure of an external circuit, an image signal amplifiercircuit 705 which amplifies an image signal included in signals receivedby a tuner 704, an image signal processing circuit 706 which convertsthe signals outputted from the image signal amplifier circuit 705 intochrominance signals corresponding to colors of red, green, and blue, acontrol circuit 707 which converts the image signal into an inputspecification of a driver IC, and the like are provided on an input sideof the image signal. The control circuit 707 outputs signals to both ascanning line side and a signal line side. In a case of digital driving,a structure in which a signal line dividing circuit 708 may be providedon the signal line side and an input digital signal may be divided intom pieces to be supplied may be employed.

An audio signal in signals received by the tuner 704 is sent to an audiosignal amplifier circuit 709 and an output therefrom is supplied to aspeaker 713 through an audio signal processing circuit 710. Acontrolling circuit 711 receives control information such as a receivingstation (reception frequency) or sound volume from an input portion 712and transmits signals to the tuner 704 or the audio signal processingcircuit 710.

Such a liquid crystal display panel module is incorporated into eachchassis as shown in FIGS. 21A to 21C, whereby a television device can becompleted. In FIG. 21A, a main screen 2003 is formed by a display moduleand a speaker portion 2009, an operation switch, and the like areprovided as accessories. In the foregoing manner, a television devicecan be completed by the present invention.

A display panel 2002 is incorporated in a chassis 2001. The televisiondevice can receive general TV broadcast by a receiver 2005 and furthercan be connected to a wired or wireless communication network via amodem 2004 so that one-way (from a sender to a receiver) or two-way(between a sender and a receiver or between receivers) informationcommunication can be carried out. The television device can be operatedby a switch of the chassis or a separate remote control unit 2006. Theremote control unit may have a display portion 2007 which displaysinformation to be outputted.

The television device may include a sub screen 2008 formed using asecond display panel for display channels, volume, and the like, inaddition to the main screen 2003. In this structure, the main screen2003 and the sub screen 2008 may be formed with use of a panel for aliquid crystal display of the present invention. Alternatively, the mainscreen 2003 may be formed using a panel for an EL display having asuperior viewing angle, and the sub screen may be formed using a panelfor a liquid crystal display which can display an image with lower powerconsumption. Note that in order to reduce the power consumptionpreferentially, a structure in which the main screen 2003 is formed of apanel for a liquid crystal display, and the sub screen is formed of apanel for an EL display and can blink may be employed. With the presentinvention, a highly reliable display device can be manufactured even ifa large substrate and a number of TFTs is and electronic parts are used.

FIG. 21B shows a television device having a large display portion of 20to 80 inches, for example. The television device includes a chassis2010, a key board portion 2012 which is an operation portion, a displayportion 2011, a speaker portion 2013, and the like. The presentinvention is applied to manufacturing of the display portion 2011. Sincea substance which can be curved is used in the display portion in FIG.21B, a television device having a curved display portion is formed. Insuch a manner, a shape of a display portion can be freely designed;therefore, a television device with a desired shape can be manufactured.

FIG. 21C shows a television device having a large display portion of 20to 80 inches, for example. The television device includes a chassis2030, a display portion 2031, a remote control unit 2032 which is anoperation portion, a speaker portion 2033, and the like. The presentinvention is applied to manufacturing of the display portion 2031. Thetelevision device shown in FIG. 21C is a wall-hanging type and does notrequire a large installation space.

It is needles to say that the present invention is not limited to atelevision device. The present invention can be applied to variousapplications such as a monitor of a personal computer, and particularly,a large display media typified by an information screen at trainstations, airports, or the like, and an advertising display screen onthe street.

Embodiment Mode 8

As electronic appliances relating to the present invention, a televisiondevice (also simply referred to as a television or a televisionreceiver), a camera such as a digital camera and a digital video camera,a mobile phone device (also simply referred to as a mobile phone), aportable information terminal such as a PDA, a portable game machine, amonitor for a computer, a computer, an audio reproducing device such asa car audio set, an image reproducing device provided with a recordingmedium such as a home game machine, and the like can be given. Specificexamples are described with reference to FIG. 22A to 22E.

A portable information terminal shown in FIG. 22A includes a main body9201, a display portion 9202, and the like. The display device of thepresent invention can be applied to the display portion 9202. Thus, ahigh performance portable information terminal which can display a highquality image with high reliability can be provided.

A digital video camera shown in FIG. 22B includes a display portion9701, a display portion 9702, and the like. The display device of thepresent invention can be applied to the display portion 9701. Thus, ahigh performance digital video camera which can display a high qualityimage with high reliability can be provided.

A mobile phone shown in FIG. 22C includes a main body 9101, a displayportion 9102, and the like. The display device of the present inventioncan be applied to the display portion 9102. Thus, a high performancemobile phone which can display a high quality image with highreliability can be provided.

A portable television device shown in FIG. 22D includes a main body9301, a display portion 9302, and the like. The display device of thepresent invention can be applied to the display portion 9302. Thus, ahigh performance portable television device which can display a highquality image with high reliability can be provided. In addition, adisplay device of the present invention can be widely applied, as atelevision device, to a small television, device mounted on a portableterminal such as a mobile phone, a medium one which can be carried, anda large one (for example, 40 inches or more).

A portable computer shown in FIG. 22E includes a main body 9401, adisplay portion 9402, and the like. The display device of the presentinvention can be applied to the display portion 9402. Thus, a highperformance portable computer which can display a high quality imagewith high reliability can be provided.

As described above, with a display device of the present invention, ahigh performance electronic appliance which can display a high qualityimage with high reliability can be provided.

Embodiment 1

In this embodiment, characteristics of a light emitting diode used inthe present invention was measured and evaluated. Experimental result isdescribed with reference to FIGS. 10A, 10B, and 12A to 13.

Emission spectra of light emitting diodes of RGB were measured with aspectrophotometer device which can measure a spectroscopy spectrum fromultraviolet to near-infrared. The result of the measurement is shown inFIGS. 10A and 10B. In FIGS. 10A and 10B, FIG. 10B shows a result of anemission spectrum with use of a light emitting diode which emits normalblue light, and FIG. 10A shows an emission spectrum with use of a darkblue light emitting diode which emits dark blue light, instead of theblue light emitting diode used in FIG. 10B. In FIGS. 10A and 10B,wavelength spectra of the blue light emitting diode and the dark bluelight emitting diode are denoted by a dotted line, a wavelength spectrumof a green light emitting diode is denoted by a solid line, and awavelength spectrum of a red light emitting diode is denoted by adashed-single dotted line.

In a case where the dark blue light emitting diode was used, a peakposition was shifted to a short wavelength side of 455 nm from a peakposition of 470 nm in a case where the blue light emitting diode wasused. Accordingly, an area overlapped with a spectrum of the green lightemitting diode became small and the half width of the spectrum becamenarrow. Therefore, it was recognized that color purity was furtherheightened and a color reproducibility range was widened.

A back light unit using a cold cathode fluorescent tube cold cathodefluorescent tube (also referred to as CCFL: Cold Cathode FluorescentLamp) as a light source, a back light unit using RGB light emittingdiodes including the blue light emitting diode as a light source, and aback light unit using RGB light emitting diode including the dark bluelight emitting diode were driven. Two color filters with differentthicknesses (a film thickness of 1.7 μm and a film thickness of 2.5 μm)were located over each of the back light units, and chromaticity of eachcolor of RGB was measured with a color luminance meter, under acondition in which white was adjusted to the chromaticity coordinates ofthe NTSC standard. The result of the measurement of chromaticitycoordinates is shown in FIGS. 12A and 12B. In FIGS. 12A and 12B,chromaticity coordinate of NTSC is denoted by a solid line, that of theRGB light emitting diode including the blue light emitting diode isdenoted by a dashed-single dotted line, that of the RGB light emittingdiode including the dark blue light emitting diode is denoted by adotted line, and that of the CCFL is denoted by a dashed line. FIG. 12Ashows chromaticity coordinates when the color filter with a thickness of1.7 μm was used and FIG. 12B shows chromaticity coordinates when thecolor filter with a thickness of 2.5 μm was used.

When the area of the chromaticity coordinates of the NTSCC is regardedas 100%, the area of the chromaticity coordinates of a case of using thecold cathode fluorescent tube is 67 to 92%, that of a case of using theblue light emitting diode is 72 to 101%, and that of a case of using thedark blue light emitting diode is 87 to 110%. Accordingly, it wasrecognized that when the dark blue light emitting diode is used, an areaof chromaticity coordinates can become large.

As described above, when a backlight device uses a light emitting diodeas its light source, a color reproducibility range can be widened.Therefore, with use of such a backlight device, a high performancedisplay device which can display a higher quality image can bemanufactured.

Embodiment 2

In this embodiment, characteristics of a light emitting diode of thepresent invention was measured and evaluated. The experimental result isdescribed with reference to FIGS. 11A to 11C.

Change in temperature in a backlight device when a light emitting diodeblinked was measured in a case where a measure for releasing heat wasnot taken for the light emitting diode and in a case where the lightemitting diode was provided in a chassis having a thermoelectric elementaccording to the present invention and cooling was carried out by thethermoelectric element. In specific, a temperature of a back surface ofa substrate (metal core substrate), a temperature of a front surface ofthe substrate (metal core substrate), and a temperature of a vicinity ofa resin covering the light emitting diode were measured with athermocouple. The temperature change with time of the front surface ofthe substrate is shown in FIG. 11A, that of the back surface of thesubstrate is shown in FIG. 11B, and that of the vicinity of the resincovering the light emitting diode is shown in FIG. 11C. In FIGS. 11A to11C, the case where the measure for releasing heat by the thermoelectricelement was not taken is denoted by a thin line and the case where themeasure for releasing heat by the thermoelectric element was taken isdenoted by a thick line. In this embodiment, a Peltier element was usedas a thermoelectric element.

In a case where a measure for releasing heat was not taken, atemperature of the substrate was 50° C. (both the back surface and thefront surface) and that in the vicinity of the light emitting diode (thevicinity of the resin covering the light emitting diode) was 60° C. Onthe other hand, in a case where the Peltier element was used, atemperature of the back surface of the metal core substrate was 20° C.,that of the front surface of the metal core substrate was 40° C., andthat in the vicinity of the light emitting diode (the vicinity of theresin covering the light emitting diode) was 45° C. Accordingly, it wasrecognized that the temperature of the light emitting diode in usebecame low in a case where the Peltier element was used.

As described above, with the present invention, heat generation of thelight emitting diode used in the light source can be suppressed;therefore, reduction in life time and luminance, and chromaticity shiftof the light emitting diode can be suppressed. Since heat generation inthe light source is suppressed, transformation and alteration of adiffusing film, a reflective film, and a prism film can also besuppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

Embodiment 3

In this embodiment, characteristics of a light emitting diode of thepresent invention was measured and evaluated. The experimental result isdescribed with reference to FIGS. 26A to 28B.

Change in temperature in a backlight device when a light emitting diodeblinked was measured in a case where the light emitting diode wasprovided in a chassis having a thermoelectric element and cooling wascarried out by the thermoelectric element. In specific, a temperature ofa vicinity of a resin covering the light emitting diode was measuredwith a thermocouple.

Samples are sample A (A1 to A3) and sample B (B1 to B3) and they havedifferent locations of the thermoelectric elements with respect to thelight emitting diodes. FIGS. 27A to 28B show structures of sample A andsample B. In FIGS. 27A to 28B, FIGS. 27A and 28A are top views, FIG. 27Bis a cross-sectional view taking along a line X1-Y1 in FIG. 27A, andFIG. 28B is a cross-sectional view taking along a line X2-Y2 in FIG.28A. Sample A has a structure in which a thermoelectric element 802 isprovided only under a light emitting diode 801 in a chassis 800 and thelight emitting diode 801 is provided in the center of the chassis 800(FIGS. 27A and 27 Sample B has a structure in which a thermoelectricelement 822 a is provided under a light emitting diode 821 andthermoelectric elements 822 b to 822 e are provided so as to surroundfour sides of the light emitting diode 821 in a chassis 820, and thelight emitting diode 821 is not provided in the center of thethermoelectric element 822 a but near the thermoelectric elements 822 band 822 e (FIGS. 28A and 28B).

Sample A1 and sample B1 are light emitting diodes which emit red light,sample A2 and sample B2 are light emitting diodes which emit greenlight, and sample A3 and sample B3 are light emitting diodes which emitblue (royal blue).

FIG. 26A shows the temperature changes with time in the vicinity of theresin covering sample A1 and sample B1 which are light emitting diodesexhibiting red light emission, FIG. 26B shows the temperature changeswith time in the vicinity of the resin covering sample A2 and sample B2which are light emitting diodes exhibiting green light emission, andFIG. 26C shows the temperature changes with time in the vicinity of theresin covering sample A3 and sample B3 which are light emitting diodesexhibiting blue light emission. In FIGS. 26A to 26C, sample A (A1, A2,A3) is denoted by a thin line and sample B (B1, B2, B3) is denoted by athick line. In sample A and sample B, a Peltier element was used as thethermoelectric element. In all samples, a current of 0.3 ampere (A) issupplied to the light emitting diode and a current of 2 ampere (A) issupplied to the thermoelectric element.

As shown in FIGS. 26A to 26C, increase in temperature was suppressed insamples B1 to B3 in which the thermoelectric elements were provided tofive sides of the light emitting diode of the present invention so as tosurround the light emitting diode, compared with samples A1 to A3 inwhich the thermoelectric element was provided only under the lightemitting diode. Accordingly, a structure like the present invention inwhich thermoelectric elements are provided so as to surround the lightemitting diode has high heat releasing effect on the light emittingdiode.

As described above, with the present invention, heat generation of thelight emitting diode used in the light source can be suppressed;therefore, reduction in life time and luminance, and chromaticity shiftof the light emitting diode can be suppressed. Since heat generation inthe light source is suppressed, transformation and alteration of adiffusing film, a reflective film, and a prism film can also besuppressed.

Furthermore, change in characteristics of the liquid crystal displaypanel, such as response speed, contrast, or color unevenness can besuppressed. In addition, transformation, alteration, deterioration incharacteristics, or the like of a polarizing film, a wide view film, aretardation film, and the like used in the liquid crystal display panelcan also be suppressed. Also, since a heat sink, a heat pipe, a coolingfan, and the like are not required, the backlight device can be thinned.

Therefore, with the present invention, a highly reliable backlightdevice with less color unevenness and less luminance unevenness, and ahigh-performance and highly reliable display device including thebacklight device, which can display a high quality image, can bemanufactured.

This application is based on Japanese Patent Application serial no.2006-077879 filed in Japan Patent Office on Mar. 21, 2006, the entirecontents of which are hereby incorporated by reference.

1-47. (canceled)
 48. A semiconductor device comprising: a liquid crystaldisplay panel; and a backlight device adjacent to the liquid crystaldisplay panel, wherein the liquid crystal display panel comprises: atransistor; and a sealant, wherein the backlight device comprises: aboard; a reflective sheet over the board; a first light emitting partover the board; a second light emitting part over the board; a thirdlight emitting part over the board; a diffusing plate over thereflective sheet, the first light emitting part, the second lightemitting part and the third light emitting part; and a prism sheet overthe diffusing plate, wherein the first light emitting part, the secondlight emitting part and the third light emitting part each comprises: aplurality of light emitting diodes; and a fluorescent material over theplurality of light emitting diodes, wherein the transistor overlaps withthe first light emitting part, and wherein the sealant does not overlapwith the first light emitting part, the second light emitting part andthe third light emitting part.
 49. A semiconductor device according toclaim 48, wherein the first light emitting part, the second lightemitting part and the third light emitting part emit red light, greenlight and blue light, respectively.
 50. A semiconductor device accordingto claim 48, wherein the first light emitting part, the second lightemitting part and the third light emitting part each emits white light.51. A semiconductor device according to claim 48, wherein colors of theplurality of light emitting diodes of the first light emitting part aredifferent each other.
 52. A semiconductor device according to claim 48,wherein the transistor does not overlap with the second light emittingpart.
 53. A semiconductor device according to claim 48, wherein thetransistor does not overlap with the third light emitting part.
 54. Anelectric appliance comprising a semiconductor device according to claim48, wherein the electric appliance comprises one selected from the groupconsisting of a portable information terminal, a camera, a mobile phone,a television device, and a computer.
 55. An electric appliancecomprising a semiconductor device according to claim 48, a speaker and aremote control unit.
 56. A semiconductor device comprising: a liquidcrystal display panel; and a backlight device adjacent to the liquidcrystal display panel, wherein the liquid crystal display panelcomprises: a pixel electrode layer; and a sealant, wherein the backlightdevice comprises: a board; a reflective sheet over the board; a firstlight emitting part over the board; a second light emitting part overthe board; a third light emitting part over the board; a diffusing plateover the reflective sheet, the first light emitting part, the secondlight emitting part and the third light emitting part; and a prism sheetover the diffusing plate, wherein the first light emitting part and thesecond light emitting part each comprises: a plurality of light emittingdiodes; and a fluorescent material over the plurality of light emittingdiodes, wherein the first transistor overlaps with the first lightemitting part, wherein the pixel electrode layer overlaps with the thirdlight emitting part, and wherein the sealant does not overlap with thefirst light emitting part, the second light emitting part and the thirdlight emitting part.
 57. A semiconductor device according to claim 56,wherein the first light emitting part, the second light emitting partand the third light emitting part emit red light, green light and bluelight, respectively.
 58. New A semiconductor device according to claim56, wherein the first light emitting part, the second light emittingpart and the third light emitting part each emits white light.
 59. Asemiconductor device according to claim 56, wherein colors of theplurality of light emitting diodes of the first light emitting part aredifferent each other.
 60. A semiconductor device according to claim 56,wherein the liquid crystal display panel comprises a transistor, andwherein the transistor overlaps with the first light emitting part. 61.A semiconductor device according to claim 56, wherein the liquid crystaldisplay panel comprises a transistor, and wherein the transistor doesnot overlap with the second light emitting part.
 62. A semiconductordevice according to claim 56, wherein the liquid crystal display panelcomprises a transistor, and wherein the transistor does not overlap withthe third light emitting part.
 63. A semiconductor device according toclaim 56, wherein the pixel electrode layer does not overlap with thefirst light emitting part.
 64. A semiconductor device according to claim56, wherein the pixel electrode layer does not overlap with the secondlight emitting part.
 65. An electric device comprising a semiconductordevice according to claim 56, wherein the electric device is oneselected from the group consisting of a portable information terminal, acamera, a mobile phone, a television device, and a computer.
 66. Anelectric appliance comprising a semiconductor device according to claim56, a speaker and a remote control unit.
 67. A semiconductor devicecomprising: a liquid crystal display panel; and a backlight deviceadjacent to the liquid crystal display panel, wherein the liquid crystaldisplay panel comprises: a capacitor; and a sealant, wherein thebacklight device comprises: a board; a reflective sheet over the board;a first light emitting part over the board; a second light emitting partover the board; a third light emitting part over the board; a diffusingplate over the reflective sheet, the first light emitting part, thesecond light emitting part and the third light emitting part; and aprism sheet over the diffusing plate, wherein the first light emittingpart and the second light emitting part each comprises: a plurality oflight emitting diodes; and a fluorescent material over the plurality oflight emitting diodes, wherein the first transistor overlaps with thefirst light emitting part, wherein the pixel electrode layer overlapswith the third light emitting part, and wherein the sealant does notoverlap with the first light emitting part, the second light emittingpart and the third light emitting part.
 68. A semiconductor deviceaccording to claim 67, wherein the first light emitting part, the secondlight emitting part and the third light emitting part emit red light,green light and blue light, respectively.
 69. A semiconductor deviceaccording to claim 67, wherein the first light emitting part, the secondlight emitting part and the third light emitting part each emits whitelight.
 70. A semiconductor device according to claim 67, wherein colorsof the plurality of light emitting diodes of the first light emittingpart are different each other.
 71. A semiconductor device according toclaim 67, wherein the liquid crystal display panel comprises atransistor, and wherein the transistor overlaps with the first lightemitting part.
 72. A semiconductor device according to claim 67, whereinthe liquid crystal display panel comprises a transistor, and wherein thetransistor does not overlap with the second light emitting part.
 73. Asemiconductor device according to claim 67, wherein the liquid crystaldisplay panel comprises a transistor, and wherein the transistor doesnot overlap with the third light emitting part.
 74. A semiconductordevice according to claim 67, wherein the capacitor does not overlapwith the first light emitting part.
 75. A semiconductor device accordingto claim 67, wherein the capacitor does not overlap with the secondlight emitting part.
 76. An electric device comprising a semiconductordevice according to claim 67, wherein the electric device is oneselected from the group consisting of a portable information terminal, acamera, a mobile phone, a television device, and a computer.
 77. Anelectric appliance comprising a semiconductor device according to claim67, a speaker and a remote control unit.